The invention relates to a power capacitor comprising an overpressure device, sensitive to a gas release, and at least one capacitive winding coated in a polymerizable resin cast in a case.
A low-voltage electrical power capacitor is generally made up from capacitive windings formed by a coil of metallized dielectric films. In a self-healing capacitor, in the event of an internal fault resulting in perforation of the insulator, the energy dissipated instantaneously vaporizes the metallic deposit at the place where the perforation occurred. This re-establishes the insulation at this place and thus removes the fault.
In order to protect the metallic layer from aggressions of the surrounding environment, the coils are sunk in a polymerizable resin. To achieve this, one or more windings are placed in a case whose internal dimensions are greater than those of the winding, and a polymerizable resin is cast in the free space between the winding and the case.
In certain cases, notably in the event of overloads, voltage surges, or at the end of the life of the coil, self-healing does not take place. With the fault zone in the dielectric film no longer being electrically insulated, a fault in the coil develops very quickly. The heat given off is such that melting of the dielectric film occurs, followed by boiling, thereby creating a large volume of gas. Thus internal pressure in the case increases.
French Patent 2,174,904 corresponding to U.S. Pat. No. 3,831,070, uses the gas produced in the event of a voltage surge to create pressure which increases the threshold of non self-healing discharge occurrence. For this purpose a plastic resin, polymerizing slowly and impermeable to gases, is cast directly into a case made of plastic material. It is thus possible to increase the voltage that can be withstood by the capacitor.
The patent, however, does not disclose avoiding the risk of explosion of the capacitor, especially at the end of the life of the capacitor.
To avoid the case containing the capacitive winding from exploding, it is known to electrically disconnect the coil as soon as the internal pressure exceeds a certain limit fixed by the mechanical strength of the case. For this purpose, the pressure of the gas is used to actuate a mechanical system called a booster to enable the electrical power supply to the coil to be broken or short-circuited. In known devices the gas pass through the resin coating the coil and must to be routed to reach the booster.
A difficulty, however, lies in obtaining a reliable and efficient routing whatever the pressure increase kinetics. In fact, depending on how the fault in the coil develops, a very fast pressure increase can occur akin to an explosion, or little amount of gas be emitted, but a large amount of dielectric material may be melted. In the latter case a path is required which can withstand the high temperature of the melted dielectric for several minutes, the time for the pressure to reach the value necessary for the booster to operate. To overcome this difficulty, several solutions exist at present, with their advantages and drawbacks.
In certain cases the capacitor case is metallic, such as made of aluminium. The case therefore withstands temperature and is uninflammable. The malleability of the material enables a booster to be obtained at little cost, by making one or more extendible folds in the upper part of the case. A large drawback of the metal case is that it is electrically conducting. This requires electrical insulation between the case and the coil and electrical connections. This insulation is generally achieved by an internal envelope made of plastic material. In addition, a free space is generally left between this envelope and the case for routing of the gases. Moreover, the capacitor case must be connected to the ground circuit of the electrical installation.
In the document EP-A-11,347 corresponding to U.S. Pat. No. 4,283,750, the aluminium case is provided with an extendible fold, an auxiliary part made of polyamide, which adheres only slightly to resin, being disposed in the case between the extendible fold and the base-plate of the case, and acting as a securing support for one of the electrical connections of the capacitor.
Capacitors also exist whose case is made of plastic. In this case, no problem of electrical insulation arises. A drawback of the plastic case is that, on account of its low temperature withstand, a space must be arranged between the resin and the internal face of the case for routing of the gases and molten dielectric material. This space can be achieved by covering the internal face of the case with a low-density closed-cell foam. The hot gases melt this foam thereby creating an easy path to the booster. The coil coated with resin can also be arranged in an intermediate envelope made of easily meltable plastic material. The external face of this envelope comprises, for example, fins which extend in the free space between the intermediate envelope and the internal face of the case. Another drawback of plastic materials is their inflammability.
Thus, in the state of the art of the technology, whatever the material of the case, the measures taken for routing the gas complicate manufacture of the power capacitor and lead to an increased volume of the capacitor, whether it involves leaving inside the case, generally on its internal face, a free space or a provisionally closed space. These two drawbacks have a non-negligible impact in terms of manufacturing cost.